1. Field of the Invention
This invention relates generally to bonding apparatus and is concerned more particularly with a test bonding apparatus having means for consistently heating sample sandwiched layers of thermosetting adhesive material.
2. Discussion of the Prior Art
In the production of plastic assemblies, each of the assemblies may have component parts joined to one another by interposed layers of thermosetting adhesive, such as epoxy bonding material, for example. The bonding properties of the adhesive may be tested prior to making a production run by placing a sample layer of the adhesive between overlapping portions of two relatively small plastic sheets or panels which generally are referred to as "coupons". Thus, the overlapping portions of the coupons and the sample layer of adhesive therebetween comprise a test sandwich assembly for studying the interlocking joint formed by the cured adhesive. From the study of this interlocking joint, conclusions may be drawn regarding optimal operating conditions for making the production run of the larger plastic assemblies.
The test sandwich assembly may be clamped between two pressure plates which are urged toward one another by suitable fastener means, such as screws passing through both of the pressure plates, for example. The clamped sandwich assembly then may be placed in a heated environment, such as an oven, for example, whereby heat energy is transmitted to the sample layer of adhesive between the two coupons. As a result, there is formed between the overlapping portions of the two coupons a cured adhesive joint which may be subjected to reliability tests, such as shear tests, for example.
However, it may be found that the test results, thus obtained, are not repeatable due to inconsistent heating of the adhesive layer in the oven. Consequently, a second test sandwich assembly similar to the first sandwich assembly and heated in the same oven for an equal interval of time may produce an adhesive joint which is more or less reliable than the adhesive joint achieved with the first sandwich assembly. Also, it may be found that attempts to provide an enclosure with means for consistently heating the adhesive layer may result in access to the interior of the enclosure being rendered difficult. As a result, considerable time may be consumed in placing a test sandwich assembly within the enclosure and in removing the test sandwich assembly from the enclosure.